Glioblastoma multiforme (GBM) is an almost uniformly fatal brain cancer. The current treatment for GBM is radiation with oral chemotherapy. GBM patients treated with chemotherapy and radiation have life expectancies of only 14 months. These statistics make it obvious that better treatment approaches are needed to improve the effectiveness of radiation for GBM. Radiation causes DNA damage which activates signaling pathways that are thought to improve the ability of cells to repair DNA damage. The damage sensing pathways that are triggered by radiation, for the most part, are dependent on protein phosphorylation (adding a phosphate group to proteins). Protein phosphorylation reflects the balance between two opposing sets of enzymes: kinases that add a phosphate group to proteins and phosphatases that remove a phosphate group from proteins. Drugs that block kinases have already been successfully used to improve the clinical effectiveness of radiation therapy. For example, advanced-stage head and neck cancers are treated with radiation in addition to a drug that blocks kinases. At this time, however, protein phosphatases have received little attention but have the potential to make radiation more effective. We have discovered a specific role for a newly discovered protein phosphatase called phosphatase-6 (PP6) which plays a critical role in regulating a key DNA repair protein called DNA-PK. Depletion of PP6 dramatically sensitizes glioblastoma cells to radiation. Our long term goal is to make glioblastoma cells more sensitive to radiation through the use of drugs that block PP6's ability to activate DNA-PK. However, before such drugs can be used, it is necessary to define the molecular mechanisms by which PP6 regulates DNA-PK following radiation.